Constraint?

An image of a gap style go nogo gauge, for external
                    measurements (such as the diameter of a shaft being produced by a lathe). It
                    looks sort of like a small C-clamp with two jaws (instead
                    of just one).

Gauge photograph taken by Glenn McKechnie, 2005-03-26, available via CC-by-sa-2.0 license.

Overview of TEI Customization, slide 2 of 13